Most metals occur in nature in oxidized form in their ores and thus must be reduced to their metallic forms. The ore is dissolved following some preprocessing in an aqueous electrolyte or in a molten salt and the resulting solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution.
Several metals are naturally present as metal sulfides; these include copper, lead, molybdenum, cadmium, nickel, silver, cobalt and zinc. In addition, gold and platinum group metals are associated with sulfide base metal ores. Most metal sulfides, or their salts, are electrically conductive and this allows electrochemical redox reactions to efficiently occur in the molten state or in aqueous solutions. Thus, industrial electrolytic processes have developed and have been used for over one hundred fifty years to obtain elemental metal from a molten ore or an ore in an aqueous solution.
Molten ores are formed in smelting furnaces that reach very high temperatures and require excessive amounts of fuel and energy, so most metal recovery processes use aqueous solution extraction. Unfortunately, the aqueous extraction solutions usually contain toxic, caustic and harsh chemicals, such as cyanide, hydrochloric acid, sulfuric acid that form water soluble complexes with metals.
Within the past fifty years, cyanide has remained one of the most common reactants in the processes for recovery of metals from ore. Cyanide has the great ability to form water soluble complexes with metals. It achieves this via the fact that the cyanide molecule has what is called a triple bond between the carbon and nitrogen atoms. This special bond makes the cyanide molecule very reactive and it easily forms cyano-metallic complexes such as potassium gold cyanide (KAu(CN)2), sodium zinc cyanide (Na2Zn(CN)4) and sodium copper cyanide (Na3Cu(CN)4).
Hydrogen cyanide (HCN) is converted to sodium cyanide (NaCN) for mining purposes and shipped to the mine site, usually in briquette form. The solid cyanide briquettes are added to a tanked mixture of finely crushed ore and water let stir for a period of many hours. The solid cyanide dissolves in the water portion of the mixture, attacks or leaches the metal in the ore, and forms the water soluble complex. Thus, the metal has been extracted from a solid state (in the ore) to a liquid state (in the solution).
After a metal has been put in solution or liquefied, it is subjected to electrowinning, also called electrorefining or electroextraction. Electrowinning is the electrodeposition of metals from their ores onto a plate; this is an important technique that allows purification of a non-ferrous metal in an economical and straightforward step.
Various patents claim improvements in metal recovery and electrorefining processes developed since 1865 when a commercial process for electrolytic copper refining was patented by James Elkington.
U.S. Pat. No. 5,232,490 to Bender et al. uses an oxidation/reduction process for recovery of precious metals, such as silver and gold, from manganese dioxide ores, sulfidic ores and carbonaceous materials. The process involves teaching the ore with a leach liquor comprising an acid, such as hydrochloric acid (HCl) and sulfuric acid in the presence of a reductant to dissolve the precious metals. Recovery of the dissolved precious metals in the fluid can be by electrolysis.
U.S. Pat. No. 5,205,858 to Manke describes a precious metals recovery process using the standard cyanide-extraction technique together with carbon adsorption to facilitate the recovery.
U.S. Pat. Publ. No. 2003/0039605 A1 to Ramsay discloses a process for recovering precious metals from fine carbon bearing residual amounts of precious metals. The process involves incinerating carbon followed by a method for separating the precious metals from carbon ash; separation could include cyanidation, gravity concentration, smelting, electrowinning and solvent extraction.
U.S. Pat. No. 6,972,107 B2 to Marsden et al. describes a system for direct electrowinning of copper from a leach solution of a copper-containing ore, concentrate, or other copper-bearing material without the use of copper solvent extraction techniques or apparatus.
None of the prior art processes for recovery of metals from ore use a non-toxic, non-caustic composition to extract metal from its ore. All of the processes have complicated, hazardous, costly processing steps. Many of the processes have disastrous consequences to the environment where the processes are performed. There is a need for a non-toxic, environmentally friendly, efficient process for recovering metals from ore that also conserves natural resources. The present invention fulfills this need.